The invention relates generally to an apparatus for controlling the operational fuel-air mixture fed to an internal combustion engine. More particularly, the invention relates to an apparatus for controlling the operational mixture on the basis of a variable derived from the prevailing exhaust gas composition of the engine. This variable is transduced by a socalled oxygen or .lambda. sensor which is located in the exhaust system of the engine and which generates a signal that permits deductions regarding the composition of the fuel-air mixture fed to the engine with respect to relative richness or leanness. The adjustment of the mixture on the basis of the .lambda. sensor signal is superimposed on a basic control signal derived from signals related to engine speed (rpm) and the air flow rate, on the basis of which a fuel metering device, for example a carburetor, or intermittent or continuously injecting fuel injection systems provide fuel to the engine, preferably through electromagnetic injection valves. The invention also relates to systems in which the fuel guantity is derived from engine speed and air flow rate and is supplied continuously in dependence of a control pressure. The fuel control system includes a circuit which generates a reference voltage which is to be compared with the sensor signal in a comparator circuit behind which is connected an integrating circuit which forms the correction signal for changing the fuel quantity on the basis of exhaust gas composition.